AN EXPERIMENTAL-STUDY OF SUPERSONIC MICROJETS

Miniature axisymmetric supersonic nozzles were produced with exit Mach numbers ranging from 1.0 to 2.8 by forming Pyrex(R) capillary tubing of 0.6 and 1.2 mm inside diameter into converging-diverging channels. The nozzle contours were measured and were found to compare favorably to ideal solutions given by the axisymmetric method of characteristics . In addition, the surfaces of these nozzles were quite smooth, provid ing featureless flows at perfect expansion. Schlieren visualization an d pitot pressure measurements of the resulting microjets were compared to the literature available for jets produced by larger-scale nozzles . A postponed transition to turbulence is noted in these microjets due to their low Reynolds number. The pitot pressure on centerline is nea rly uniform at perfect expansion over core lengths up to 12 nozzle exi t diameters. Supersonic microjet nozzles thus provide a more effective small-scale high-pressure gas delivery device than do sonic nozzles o f comparable scale at equivalent mass flow rates. Supersonic microjets may therefore have several industrial applications.